Refactor the HTTP Client

[RobertoPrevato]

• Add native HTTP/2 support to the HTTP client with automatic protocol detection
  via ALPN (Application-Layer Protocol Negotiation). The client now automatically
  uses HTTP/2 when the server supports it, with seamless fallback to HTTP/1.1.
• Add new HTTP2Connection class using the h2 library for HTTP/2 protocol handling.
• Add new HTTP11Connection class using the h11 library for consistent HTTP/1.1 handling
• Both connection types use asyncio.open_connection streams for a unified architecture.
• HTTP/2 connections support stream multiplexing, allowing multiple concurrent requests
  over a single TCP connection.
• Add http2 parameter to ClientSession (defaults to True) to control HTTP/2 usage.
• Protocol detection is performed once per host and cached for efficiency.
• Add h2>=4.0.0,<5.0.0 as a new dependency.
• Refactor HTTP/1.1 client implementation to use the h11 library instead of
  custom request writing methods from blacksheep.scribe. This provides a more
  robust and standards-compliant HTTP/1.1 state machine, and creates consistency
  with the HTTP/2 implementation pattern.
• The new HTTP11Connection class replaces the asyncio.Protocol-based approach
  with a streams-based implementation.
• Remove the legacy ClientConnection class has been removed.
• Both HTTP/1.1 and HTTP/2 implementations follow the same architectural pattern.
• Remove the option of passing the event loop to the constructor of client classes.